We will study two brain regions that are structurally different in male and female rats. These examples of sexual dimorphism offers a means of studying the relationship between neuroanatomical measures and behavior. Any sexual dimorphism that is found in the brain could potentially contribute to sex differences in behavior. We will focus upon the hippocampal formation, which has been implicated in spatial cognition in mice, rats and humans. Spatial cognitive abilities have also been found to differ between the two sexes in all three species. We have recently replicated previous reports of sexual dimorphism of this structure in all three species. We have recently replicated previous reports of sexual dimorphism of this structure in rats and will apply our expertise in behavioral endocrinology to determine when the sexual dimorphism arises, whether hormones play an important role in the masculinization of the structure and, if so, which hormone receptor is activated in vivo. We will also examine the medial amygdala, which has been implicated in both masculine copulatory behavior and juvenile play behavior in rats. We recently replicated the reported sexual dimorphism in the overall volume of this nucleus but were surprised to find that it was feminine in rats with a defective androgen receptor. As with the hippocampus, we will examine when and how the medial amygdala comes to develop differently in male and female rodents. Such analyses may offer insights into possible mechanisms underlying sex differences in human behavior and psychopathology.